Responses
i think he means that, to be able to withstand the attack of kai kingdom (by defeating xing kingdom), he would hurt Yona (by not helping her and saving her friends)
i mean he the king he was speaking about min soo the attendant since his image was their !
http://www.mangago.me/read-manga/akatsuki_no_yona/an/manga/akatsuki_no_yona/c138/pg-16/ what he means by this :( and to that end ... you would even inflict even more suffering upon that person )
nb : I don't think soo won killed the ex king ! it was a big misunderstanding ! the princess didn't see the act of Murder ! only soo won with the weapon in his hand covered in blood.